Not applicable.
Not applicable.
The present invention generally relates to medical diagnostic imaging systems, and in particular relates to resolution calibration in medical imaging systems employing digital detectors.
X-ray imaging has long been an accepted medical diagnostic tool. X-ray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial, and abdominal images that often include the information necessary for a doctor to make an accurate diagnosis. When having a thoracic X-ray image taken, for example, a patient stands with his or her chest against an X-ray sensor as an X-ray technologist positions the X-ray sensor and an X-ray source at an appropriate height. The X-ray sensor then detects the X-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system (where the X-ray sensor is a solid digital image detector) scans out the detected X-ray energy and prepares a corresponding diagnostic image on a display. If the X-ray sensor is conventional film, the film is subsequently developed and displayed using a backlight.
In any imaging system, X-ray or otherwise, image quality is of primary importance. In this regard, X-ray imaging systems that use digital image detectors (xe2x80x9cdigital X-ray systemsxe2x80x9d), face certain unique difficulties. In particular, digital X-ray systems must meet stringent demands on Critical to Quality (CTQ) measurements including image resolution (both in an absolute sense, and in uniformity across an image), image resolution consistency (e.g., from system to system and over time), and image noise. In the past, however, digital X-ray systems were often unable to meet CTQ parameters or provide consistent image quality across detectors, in part due to process variations in the semiconductor fabrication techniques used to manufacture solid state digital image detectors, or inherently due to the imaging technology.
Thus, for example, two different digital X-ray systems at a single location may have a noticeable variation in perceived image quality, even though both systems pass the CTQ measurements. Doctors or technicians may then unnecessarily consider one machine inferior to another, refrain from using one or more capable machines, or spend time trying to resolve image differences between the two digital X-ray systems. Furthermore, the digital X-ray system provider may incur the time and expense of responding to maintenance calls to address the perceived digital X-ray system variation, only to find that both digital X-ray systems are within specification (or to incur great expense of time and money to find a digital image detector for the perceived inferior machine that matches the perceived superior system).
As noted above, the characteristics of digital image detectors inherently vary. Although there is a need to provide consistent image quality (and in particular, image resolution) across multiple digital imaging systems, there has been in the past no automated technique for providing such consistency. Furthermore, the stringent CTQ measures may result in low acceptable yields for digital image detectors which are then destroyed, or, at best, unusable for medical diagnostic systems. Consequently, time, money, and resources are wasted.
A need has long existed in the industry for a method and apparatus for providing control over resolution in digital imaging systems that overcome the problems noted above, and others previously experienced.
A preferred embodiment of the present invention provides a method and apparatus for calibrating resolution of a digital imaging system. The method and apparatus include measuring unadjusted performance of a first digital image detector used in the imaging system. The method and apparatus then determine a weighting coefficient for a first spatial frequency band processed by the medical imaging system. Each spatial frequency bands processed may, for example, be formed using a multiresolution decomposition technique, such as a Burt pyramid decomposition. The weighting coefficient is based on a predetermined desired performance of the digital image detector and the measured performance of the digital image detector. The weighting coefficient may also be based on a target performance or target resolution. The weighting coefficient is stored for subsequent application to the spatial frequency band by the medical imaging system. Identical or distinct weighting coefficients may be used at multiple spatial resolution levels. A single weighting coefficient may be applied to all pixels at a given spatial resolution, or numerous spatial resolution variation compensation coefficients may be used in different regions of each spatial frequency band. The calibration may be generalized to continuous time systems using a calibration factor rather than discrete weighting coefficients.
The preferred embodiment also provides a resolution calibration subsystem for a digital imaging system employing a digital image detector. The resolution calibration subsystem includes a memory storing digital image data (e.g., image data acquired from a digital image detector such as a solid state X-ray detector) and a processor coupled to the memory. The processor, through dedicated hardware, or a hardware/software combination determines a first spatial frequency band of the image data and applies a weighting coefficient to the first spatial frequency band. The weighting coefficient is based on a predefined desired resolution (for example, expressed in terms of a predefined desired Modulation Transfer Function) of the digital image detector and a measured resolution (for example, in terms of a measured Modulation Transfer Function) of the digital image detector. Again, the calibration may be generalized to continuous time systems using a filter with frequency response characterized by the calibration factor.